This invention relates to an amplifier circuit, in particular but not exclusively to an amplifier circuit for providing bandpass amplification at intermediate frequencies in radio receivers.
Amplifiers are widely used in the prior art for amplifying input signals applied thereto to provide amplified output signals. This is particularly important in radio receivers in which radiation received thereat generates corresponding antenna received signals which typically have an amplitude of microvolts. The radio receivers employ amplifiers therein to amplify such received signals to an amplitude in the order of millivolts to volts, for example to drive a loudspeaker. Since it is difficult to prevent amplifiers designed to amplify at radio frequencies from spontaneously oscillating, especially if they comprise cascaded gain providing stages, it is customary to heterodyne the received signals to lower intermediate frequencies whereat it is easier to provide a high degree of amplification and also provide more selective bandpass signal filtration.
In prior art radio receivers, it is therefore customary to provide a majority of signal amplification required at intermediate frequencies, namely frequencies lying intermediate between that of the radiation received and audio or video frequencies. For example, a radio receiver receives radiation at a frequency of 500 MHz and generates a corresponding antenna received signal also at 500 MHz. The receiver heterodynes the received signal to generate an intermediate frequency signal in a frequency range around 10.7 MHz which is then amplified and filtered, and finally demodulates the amplified intermediate frequency signal to generate a corresponding audio output signal having signal components in a frequency range of 100 Hz to 5 kHz.
Recently, because the radio frequency spectrum is becoming increasingly congested, there is a trend to use an ultra high frequency (UHF) range in contemporary communications systems, namely around 500 MHz; transmission at microwave frequencies, for example 1 GHz to 30 GHz is now also employed. Associated with this is a trend in modern radio receiver design to employ intermediate frequency amplification at several tens of MHz or greater, this is done in order to obtain adequate ghost image rejection associated with using heterodyne processes.
In modem mobile telephones, most signal amplification is provided in intermediate frequency amplifier circuits incorporated therein. These circuits comprise transmission amplifiers and associated surface acoustic wave (SAW) or ceramic filters to provide a narrow bandpass signal amplification characteristic; the circuits and their associated filters are conventionally referred to collectively as an xe2x80x9cintermediate frequency stripxe2x80x9d. Such transmission amplifiers consume significant power in operation, for example intermediate frequency amplifier circuits employed in mobile telephones typically consume between several hundred microamperes and several mA of current when operational.
In order to provide modern mobile telephones with extended operating time from their associated batteries, new types of battery have been researched and developed which provide enhanced charge storage to weight performance, for example rechargeable metal hydride and lithium batteries.
The inventor has appreciated, rather than concentrating on improving battery technology, that reduction in current consumption of intermediate frequency amplifier circuits in radio receivers is desirable to provide extended operating time from batteries. The invention has therefore been made in a endeavour to provide an alternative type of amplifier circuit, for example a circuit especially suitable for use at intermediate frequencies in radio receivers which is capable of requiring less power to operate.
It is known in the art, as described in a Japanese patent application no. JP 55137707, to cascade reflection amplifiers in series and interpose filters therebetween to prevent higher harmonic components generated in preceding stages from propagating to successive stages. The filters are not operable to inhibit signal propagation in a reverse direction along the cascaded series of amplifiers to prevent the occurrence of spontaneous oscillation.
According to the present invention, there is provided an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit characterised in that it comprises:
(a) a plurality of reflection amplifiers cascaded in series along a signal path and operative to amplify the input signal propagating in a forward direction therealong to provide the output signal; and
(b) connecting means for connecting the reflection amplifiers to form the signal path and for hindering signal propagation in a reverse direction therealong, thereby counteracting spontaneous oscillation from arising within the circuit, the connecting means incorporating filters which are interposed between neighbouring reflection amplifiers along the signal path, and modulating means for modulating the input signal to associated sideband signal components and converting to and from the sideband components along the path, the filters and the modulating means operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong.
This provides the advantage that interposition of the filters between the amplifiers is capable of isolating each amplifier from its neighbouring amplifiers, thereby hindering signal propagation in the reverse direction along the path; incorporation of the modulating means enables the input signal propagating through each amplifier to be converted between a carrier and a sideband signal, thereby enabling it to propagate through the filters in the forward direction along the path.
The circuit provides the benefit that is capable of providing signal amplification and consuming less current during operation compared to prior art amplifier circuits.
One skilled in the art would not expect it to be practicable to connect a plurality of refection amplifiers together and obtain stable amplification therefrom because of spontaneous interfering oscillations which would arise during operation. The circuit addresses this problem by incorporating the connecting means which promotes intended signal amplification in the circuit and counteracts signal amplification giving rise to spontaneous oscillation therein.
Spontaneous oscillation is defined as self induced oscillation arising along a signal path providing amplification as a consequence of feedback occurring around or within the signal path.
Conveniently, the filters are arranged in series along the signal path, the filters alternating between sideband transmissive filters and sideband rejective filters along the path, and the modulating means is arranged to convert the input signal as it propagates along the signal path alternately between a corresponding carrier signal transmissible substantially through the sideband rejective filters only and a corresponding sideband signal transmissible substantially through the sideband transmissive filters only, thereby promoting input signal propagation in the forward direction along the path and hindering signal propagation in the reverse direction therealong.
In another aspect, the invention provides a method of amplifying an input signal and providing a corresponding amplified output signal, the method characterised in that it includes the steps of:
(a) providing a plurality of reflection amplifiers cascaded in series along a signal path, and connecting means for connecting the reflection amplifiers to the signal path and operative to promote signal propagation in a forward direction along the path and counteract signal propagation in a reverse direction therealong, the connecting means incorporating filters which are interposed between neighbouring reflection amplifiers along the signal path, and modulating means for modulating the input signal to associated sideband signal components and converting to and from the sideband components along the path, the filters and the modulating means operative to promote signal propagation in the forward direction along the path and hinder signal propagation in the reverse direction therealong;
(b) receiving the input signal at the signal path;
(c) directing the input signal through the connecting means to one of the reflection amplifiers for amplification therein to provide an amplified signal;
(d) directing the amplified signal to another of the reflection amplifiers for further amplification therein;
(e) repeating step (d) until the amplified signal reaches an output of the signal path; and
(f) outputting the amplified signal as the output signal from the signal path.
The method provides the advantage that, during amplification, the signal is selectively directed from amplifier to amplifier in a forward direction along the signal path, thereby counteracting any of the amplifiers reamplifying the input signal and hence preventing any feedback loops being established in which spontaneous oscillation can arise.